1. FIELD OF THE INVENTION
This invention relates to the pressing of flexible discs and, more particularly, to the pressing of optically readable video discs.
2. DESCRIPTION OF THE PRIOR ART
It is known that "flexible" audio discs can be pressed by means of compression presses comprising heating plates of which one carried the engraved matrix. The discs are obtained by thermoforming a thermoplastic material according to a predetermined pressure-temperature cycle. During this cycle, the heating plates are subjected to stresses which give rise to deformations remaining in the disc thus obtained after cooling. The contact systems used for reading audio discs are such that the contact pressure applied to the diamond causes it to follow the surface irregularities of the disc. Accordingly, the tolerances concerning the surfaces of discs such as these are fairly wide.
By contrast, the dimensions of the elements recorded on the surface of an optically readable video disc and the reading systems used for optically reading the recorded elements of such discs impose much more stringent requirements on the production of the discs, particularly in regard to their surface state, and call for a certain number of precautions to be taken during pressing. It is known for example to use a compression press in which the material to be pressed is held between a matrix, on which a negative of the information to be recorded on the flexible disc has been formed, and a counter cushion comprising at least one surface layer and an elastic pad capable of sliding on this surface layer so as to maintain a substantially uniform pressure over the entire surface to be pressed. In improved versions of this arrangement, the counter cushion is made up of an elastic pad and several layers adhering to one another by friction. Arrangements such as these enable thermoplastic sheets to be pressed by neutralising the unevenly distributed tensions which would give rise to deformation of the pressed sheets in their finished state. Arrangements such as these are difficult to produce and use on account of the multi-layer structure of the counter cushion. In addition, they are asymmetrical because the matrix rests on a rigid support.